1. Field of the Invention
This invention relates to a semiconductor device having TFTs (thin film transistors) provided on an insulating substrate of glass or the like, and a method for producing the semiconductor device.
2. Description of Related Art
TFTs have been conventionally formed on a glass substrate to form a semiconductor device such as an active matrix liquid crystal device or an image sensor. The TFTs are used, for example, to drive the pixels of the liquid crystal device.
The TFTs used in the above devices are generally formed of a silicon semiconductor layer in the form of a thin film. The silicon semiconductor of a thin-film type is classified into two types, an amorphous silicon semiconductor (a-Si) type and a crystalline silicon semiconductor type. The amorphous silicon semiconductor can be relatively easily produced at a low film-forming temperature by a vapor-phase deposition method. Therefore, this type is suitable for mass production, and it has been most generally used. However, this type of silicon semiconductor has inferior physical properties such as electrical conductivity, etc. to the crystalline silicon semiconductor. Therefore, in order to more improve a high-speed response characteristic of TFTs, a producing method for TFTs comprising crystalline silicon semiconductor has been strongly required to be established. As the silicon semiconductor having crystallinity have been known polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystal components, semi-amorphous silicon having an intermediate state between crystallinity and amorphousness, etc.
As a method for obtaining a crystalline semiconductor layer, it is well known that an amorphous silicon is initially deposited on a substrate and then heated at a high temperature.
However, this method requires a heating temperature above 600.degree. C., and thus an inexpensive glass substrate is not usable. Therefore, it is required to establish an improved method which enables the heat crystallization at a lower temperature. Particularly in case of present liquid crystal display devices, a large-area screen design is being promoted, and thus use of a large-size glass substrate is required. When a large-size glass substrate is used, contraction and distortion of a substrate occur in the heating process which is indispensable to produce crystalline semiconductors, and they cause a critical problem that the precision of a masking process is reduced. Particularly in a case of 7059 glass which is most generally used at present, it has a distortion point of 593.degree. C., and it is greatly deformed in a conventional heat crystallization method. In addition to the heat problem as described above, a heating time required for crystallization is over several tens hours in a present process, and thus the heating time must be shortened.